/Engine/Source/Runtime/GeometryCore/Public/DynamicMeshEditor.h
Duplicate triangles of a mesh. This duplicates the current groups and also any attributes existing on the triangles.
Make a copy of provided triangles, with new vertices. You provide IndexMaps because you know if you are doing a small subset or a full-mesh-copy.
void DuplicateTriangles ( const [TArray](API\Runtime\Core\Containers\TArray)< int > & Triangles, [FMeshIndexMappings](API\Runtime\GeometryCore\DynamicMesh\FMeshIndexMappings) & IndexMaps, [FDynamicMeshEditResult](API\Runtime\GeometryCore\FDynamicMeshEditResult) & ResultOut )